Field of the Invention
The invention relates to a method and magnetic resonance scanner for acquiring a magnetic resonance data set of a plurality of slices of a volume of interest of an examination object (patient).
Description of the Prior Art
For the acquisition of magnetic resonance signals, it is known upon, receiving a trigger signal relating to the patient's respiration and indicating the start of an acquisition time window, to begin a fat saturation technique and, to acquire magnetic resonance signals for the different slices in one of multiple acquisition blocks. These blocks include echo trains each relating to a single slice and a single portion of k-space, and all the echo trains of each individual acquisition block relate to different slices. A magnetic resonance image of each slice is determined by combining the magnetic resonance data acquired in different acquisition blocks and relating to a portion of k-space for that slice.
Magnetic resonance imaging is well established as a clinical imaging method. Acquisition techniques exist in which only the magnetic resonance signal received from water bound protons (water signal) is to be contained in the magnetic resonance data set obtained, but not magnetic resonance signals arising from fat bound protons (fat signals). Various fat saturation techniques are therefore known in which, for example, a fat saturation pulse is emitted at regular intervals as a radiofrequency (RF) pulse.
In addition, magnetic resonance sequences are known in which it is impossible or, because of the image quality, undesirable, to capture the entirety of k-space to be acquired after a single excitation pulse, because excessively severe signal decay occurs. In this context it is known to carry out so-called “multi-shot acquisitions” in which the magnetic resonance sequence is subdivided into a number of echoes or echo trains that differ in their phase encoding portions such that different portions of k-space are acquired by each of these echo trains. In this context, a number of slices are frequently acquired, so that during the execution of echo trains for other slices, the signal of the previously acquired slice can relax again and a new excitation pulse can be generated in the next echo train in relation to that slice. With regard to the fat saturation to be performed, this means that the fat saturation technique is begun using the first echo train so that, for example, the fat saturation pulse is emitted prior to the start of each echo train. The saturation pulse is not slice selective and affects the entire volume of interest. Since a number of fat saturation pulses may be required in order to achieve the desired result, it can happen that sufficient fat saturation is not yet present for the first echo trains, which is not a problem, however, as long as the other portions of k-space are acquired with longer running, sufficient fat saturation for the slices in question.
Problems arise whenever the multi-shot acquisitions are carried out in a triggered manner, in particular by a trigger signal relating to the respiration of the examination object, i.e. of a patient, and indicating the start of an acquisition time window. It has been demonstrated that, in a patient's respiratory cycle, acquisition time windows exist that show only slight movements in the volume of interest affected by respiration. Their start is marked by a particular point in the patient's respiratory cycle, which point is indicated by the trigger signal. The acquisition time window can now be used to execute an acquisition block which contains a plurality of echo trains. It is known to provide an echo train relating to an acquisition block specific portion of k-space in each of the acquisition blocks for each of the slices to be acquired. When an acquisition block is complete, the system waits for the next trigger signal.
In an example involving four slices S1, S2, S3 and S4 to be acquired and a segmentation of k-space into four portions K1, K2, K3 and K4, this means, for the first acquisition block, that the portion K1 is successively acquired for the slices S1, S2, S3, S4, in the second acquisition block the portion K2 for the slices S1, S2, S3, S4, etc. The fat saturation technique is also restarted for each acquisition block, as cyclical continuation of the fat saturation technique is not automatically possible because of the unknown duration of the pauses between the acquisition blocks.
However, in the case of triggered multi-shot acquisitions with fat saturation, the result of this is that a fat signal varying across the slices can occur. The effect of this is that the first slices in an acquisition block have a very low fat saturation, whereas the other slices have a more marked fat saturation. If, for example, five slices are acquired in an acquisition block, it may occur that fat saturation is not complete for the first two slices to be acquired in an acquisition block. If, for example, twenty slices are to be acquired in this way, wherein only five echo trains, for example, are possible within the acquisition time window, the effect is repeated for each slice group, so that, for example, eight out of twenty slices exhibit no or only low fat saturation. This results in a noticeable difference in image quality for different slices.